1. Field of the Invention
The present invention is directed to processes and apparatuses for embrittling an outer protective coating of a pipe or pipeline, and in particular processes and apparatus for the continuous embrittlement of such outer protective coatings at a speed matching that of typical pipeline traveling coating removing equipment.
2. Brief Description of the Related Art
U.S. patent application Ser. No. 07/265,772, filed on Nov. 1, 1988 by Hubert et al is herein incorporated by reference. This application discloses a pipeline traveling liquid nitrogen (LN2) spraying refrigeration tunnel which enables pipeline maintenance operations to proceed faster and with complete success in removing a coating from a pipe or pipeline, thereby allowing the unimpaired inspection of the pipe for the detection of dangerous corrosion pits and, if necessary, the selection of pipe sections that need to be replaced, in addition to providing a pipe surface clean enough for recoating.
The process and apparatus described in Hubert et al emphasize the simplicity of the LN2 tunnel, its incorporation into the typical pipeline traveling equipment, and its high speed of refrigeration. That speed, necessary to permit the tunnel of the length disclosed in Hubert et al to travel at the same speed as the typical pipeline traveling equipment, was achieved through spraying the coated pipe with a large number of wide, full cone nozzles, densely packed, uniformly distributed and close to the surface of the coated pipe, which resulted in very high heat transfer coefficients.
However, the high heat transfer coefficients of Hubert et al were achieved at the cost of low efficiencies. That is, a high average refrigeration speed (i.e., the average magnitude of temperature drop within the steel of the pipeline per unit dwell time; the coating experiences a much larger average temperature drop than the steel) was achieved at the expense of a high average refrigeration cost (i.e., the actual amount of LN2 necessary to drop the temperature within the steel of the pipeline on average per unit temperature and per unit area of sprayed surface) and of a low uniformity of the refrigeration field (i.e., the deviation of local temperature drops around the circumference of the pipe from the average temperature drop).
It was therefore considered to be desirable to reduce the LN2 consumption of the process and improve the uniformity of refrigeration without sacrificing, if possible, the refrigeration speed. The difficulty in satisfying all three of the above criteria is that techniques for improving one of the above criteria typically aggravate at least one of the other criteria. For example, an increase in the speed of refrigeration can be achieved by increasing the number of spraying nozzles and raising the injection pressure so as to increase the surfacic LN2 deposition rate. However, this typically increases the cost of refrigeration. For example, the flow rate of LN2 might be doubled while the refrigeration speed increases only by 50%, thereby resulting in a 50% efficiency loss.